Silicon photonics is the study and application of photonic systems that use silicon as an optical medium. A silicon photonics optical waveguide device typically includes a substrate, a cladding layer formed on the substrate, and a waveguide—also called waveguide core—formed in the cladding layer for transmitting an optical signal. Such waveguide devices are also called integrated optical devices or opto-electronic integrated circuits. The waveguide core commonly has the shape of a silicon strip or rib.
Light signals may be introduced into and out of the waveguide core by coupling optical fibers thereto. The core serves to confine an optical light signal transmitted thereto, and the light signal is mostly contained in the core as it is transmitted therethrough due to differences in refractive index of the materials that make up the waveguide core and the cladding layer.
Silicon photonics has been introduced into complementary metal-oxide semiconductor (CMOS) technology to allow simultaneous manufacture of electronic and optical components on a single integrated chip. As such, CMOS processes, such as masking and etching, are commonly used to form the waveguide core.
An optical modulator is a type of silicon photonic device. Optical modulators based on interferometric or resonant waveguide structures modulate light by introducing a change in the effective refractive index, which provides a shift in the optical phase of a lightwave passing through the modulator. This index change is often accomplished by implementing a phase shifter diode across the waveguide and operating the diode in either the forward-biased or reverse-biased condition. In the forward-biased condition, the diode injects a comparatively large amount of current at low voltage, inducing a large index shift for a given length of waveguide.